This project is devoted to the molecular cloning of the gene or genes responsible for the cytotoxicity of granules purified from rat NK-like tumore cells. Using a cDNA expression library and antiserum to the purified 60 Kd cytolytic component of the granules (termed "cytolysin"), we have obtained a cDNA clone which strongly reacts with the 60 Kd antiserum. This cDNA clone also reacts weakly with whole granule antiserum. Preliminary results indicate that the immunoreactive cDNA clone is approximately 700 nucleotides in length.